Electrical resistance element



1967 w. M. FABER, SR., ETAL ELECTRICAL RESISTANCE ELEMENT Filed Nov. l2,1963 RUO AND/OR FINELY DIVIDED VEHICLE IRO GLASS PARTICLES RESISTANCECOMPOSITION APPLICATION ONTO PREPARATION OF SUBSTRATE FIXED RESISTANCEELEMENT FIRING FIRING FIGURE FIGURE 2.

INVENTORS WILLIAM M. FABER SR.

GAYLORD L. FRANCIS CURTIS HOLMES- OTIS F. BOYKIN ATTORNEY United StatesPatent 3,304,199 ELECTRICAL RESISTANCE ELEMENT William M. Faber, Sr.,Elkhart, Gaylord L. Francis,

Berne, and Curtis L. Holmes, South Bend, Ind., and Otis F. Boykin,Chicago, Ill., assignors to CTS Corporation,

Elkhart, Ind., a corporation of Indiana Filed Nov. 12, 1963, Ser. No.322,702 11 Claims. (Cl. 117-201) This invention is acontinuation-in-part of our copending application entitled ElectricalResistor and Method of Making the Same, Serial No. 169,355, filed onJanuary 29, 1962, and now abandoned.

The present invention relates to electrical resistance elements and toresistance compositions and, more particularly, to an improvedresistance composition for producing electrical resistance element ofthe ceramic type such as a fixed volumetric resistance element or as athin film resistance element fired onto a surface of a base or substrateof high-temperature-resistant electrically nonconductive material.

It had generally been believed that the material employed to produce theconductive fraction of a ceramic resistance element should be metal inits chemically pure state, especially a metal highly resistant tooxidation. For example, Place et a1. (Patents Nos. 2,959,995 and2,959,996) combined a selected metal or metals with a powdered glassfrit, either by admixing the same in finely divided form, i.e., in apowdered state, or as larger particles, and grinding the mixture in aball mill. The metal or metals employed in the mixture were referred toas being nonreactive and nonoxidizable. In an alternative proceduredisclosed by Place et al., the powdered glass frit was mixed with theselected metal or metals in the form of soluble metal compounds whichare decomposable by heat.

DAndrea Patent No. 2,924,540 also discloses a ceramic resistorcomposition and electrical resistors produced therefrom, the resistorcomposition comprising finely divided palladium metal, with or withoutthe addition of silver, in combination with a vitreous enamel frit.Dumesnil Patent No. 3,052,573, however, points out that the DAndreaceramic resistor composition was not altogether satisfactory because theresistance obtained was too critically tied to the maturing temperatureof the enamel frit. According to Dumesnil, changing from one enamel fritto another produced widely divergent sheet resistances, inasmuch asdifferent enamel frits have different maturing temperatures. To overcomethis deficiency, Dumesnil substituted a mixture of palladium oxide orrhodium oxide for the finely divided pure metal used by DAndrea as theprincipal part of the conductive fraction of the composition.

Dumesnils use of palladium or rhodium oxide as a component of a resistorcomposition for a ceramic resistor in a sense contradicts certaindeep-rooted and universally held convictions of the prior artwhich wetoo shared until our discovery that by using an oxide or ruthenium oriridium to produce the conductive fraction of the composition, we couldproduce, and reliably reproduce, ceramic resistance elements that arefar superior in many respects-and especially in the higher resistanceranges to anything heretofore available, including those obtainable fromthe teachings of Dumesnil.

It is to be understood that the term sheet resistance, unless otherwisedefined, shall denote the resistance in ohms per square of a film ofmaterial having a thickness of approximately .001 inch.

3,304,199 Patented Feb. 14, 1967 Although some manufacturers of ceramicresistance elements have previously represented that they could supplyacceptable resistance elements in the very high resistance ranges byusing a material having a very high sheet resistance, we have, however,never been able, following the teachings of the prior art, to produceuniformly and economically ceramic resistance elements in ranges above30,000 ohms per square. Only by sufliciently elongating the resistancepath formed with a material having a low resistance per square couldceramic resistance elements with high sheet resistance values beattained.

Recognizing this limitation in the prior art sheet resistance it is,therefore, an object of the present invention to provide a ceramicresistance element produced from ruthenium oxide and/ or iridium oxidewhich can be reliably and economically manufactured with a sheetresistance in ohms per square to at least 600% higher than the sheetresistance heretofore attainable.

Another disadvantage we observed in ceramic resistance elementsheretofore available was a difiiculty in maintaining the temperaturecoefiicient of resistance at a low value, i.e., at approximately 01% perdegree centigrade or less, throughout the entire resistance range andespecially so in the resistance ranges below ohms per square and above30,000 ohms per square. Where the conductive fraction employed toproduce the resistance element was kept low in an efiort to increase thesheet resistance, the element exhibited a relatively high negativetemperature coetficient, and where the conductive fraction was higher soas to produce a sheet resistance of less than 100 ohms per square, arelatively high positive temperature coefiicient resulted.

The temperature coefficient of resistance of a ceramic resistanceelement is an extremely important consideration in present dayelectronics. If the TCR (temperature coefiicient of resistance) is toohigh, inevitable changes in ambient temperature in many modernapplications of electronic circuits could lead to serious consequences.Thus, if a resistance element with a TCR of 900 parts per million perdegree centigrade were used in a circuit subjected to a 100 C. change inambient temperature, the resistance thereof would change by a factor of9 percent or 0.09% per degree centigrade. In some circuits such a TCRcould not be tolerated. On the other hand, if a resistance element witha TCR of only one hundred parts per million per degree centigrade wereused, the change in resistance for the same change in temperature wouldbe only one percent or 0.01% per degree centigrade.

Heretofore, in order to maintain a low TCR for ceramic sheet resistanceelements having a resistance above 100 ohms and below 30,000 ohms persquare it was necessary to control several variables, e.g., the finenessof division of the metal oxide, the time and temperature of subsequentfiring or heat treating periods, and the proportion of a metal oxide toa pure metal. When prior known materials were used for preparing sheetresistance elements having a resistance above 30,000 ohms per square,the TCR would be too high and generally uncontrollable. It would,therefore, be desirable to prepare a resistance composition forproducing a ceramic resistance element having a low TCR, i.e.,approximately 100 parts per million or 0.01% per degree centigrade, forohmic values throughout the entire range of less than 100 ohms persquare to at least 180,000 ohms per square.

Accordingly, another object of the present invention is to. provide aceramic resistance element having a TCR of approximately 0.01% perdegree centigrade for ohmic ranges below 100 ohms per square and up toand at least as high as 180,000 ohms per square. We have discovered thatthis can be done reliably and predictably by using a specifiedpercentage of an oxide of ruthenium or iridium in composition forproducing a ceramic resistance element.

Extensive tests have shown that there is something unique about theoxide of ruthenium or iridium for when another metal oxide is employedto produce ceramic resistance elements of the film or the fixedvolumetric type the sheet resistance cannot be properly controlled. Whena metal oxide other than ruthenium or iridium oxide is used, a reductionin the percentage of the metal oxide present in the vitreous materialbelow a specified value in an effort to increase its sheet resistance,brings about a change in resistance so abrupt and unpredictable thatreliable consistency of the end product is well nigh impossible, and theresistance elements produced therewith have a high negative TCR.

It has, therefore, been necessary that at least 8 to of a metal oxide beadmixed with the vitreous material; otherwise there is very littlecontrol over the ohmic resistance of the finished ceramic resistanceelement. In accord with the prior art, whenever less than 8 to 10% ofthe metal oxide is employed, it is necessary to add a small percentageof a nonoxidizable metal, e.g., gold. Thus, combinations of certainmetal oxides and metals do alleviate this objectionable situation tosome extent, possibly because of the increased specific resistance whichthe combination possesses; but nothing heretofore accomplished in theceramic resistance element art can compare with the results we haveachieved by the use of an oxide of ruthenium or iridium to produce suchresistance elements. For example, when iridium dioxide is used toproduce ceramic resistance elements, we have found that by altering thepercentage of iridium dioxide in the composition, sheet resistances atleast as high as 180,000

ohms per square have been uniformly and economically reproduced. Sincethe previous resistance range did not exceed 30,000 ohms per square forceramic resistance elements having a low TCR, we have been able toincrease the resistivity by 600%. V

The only metal oxide, other than that of iridium which we have found atall suitable for high ohmic values, i.e., values in excess of 30,000ohms per square, is an oxide of ruthenium. The similarity in crystallinestructure of these two metal oxides, that is, both oxides have a rutilelattice, and the marked difference of the crystalline structure fromthat of the other metal oxides, e.g., rhodium or palladium oxide,heretofore used in the production of ceramic resistance elements toobtain precision resistance elements having a low TCR throughout a widerange no doubt explains why we have been able to achieve the superiorresults we have attained with oxides of these two metals. Since theoxides of ruthenium and iridium have substantially the samecharacteristics as concerns the purposes and objects of the presentinvention, it is to be understood that the oxide of ruthenium can bereplaced wholly or in part by the oxide of iridium throughout thespecification and claims.

In general, the present invention relates to a resistance compositionand to a ceramic resistance element wherein an oxide of ruthenium and/oriridium oxide is used to produce a resistance path within the ohmicrange of less than 100 ohms per square and up to and at least as high as180,000 ohms per square having a TCR of generally .01% per degreecentrigrade throughout the entire range.

For a better understanding 'of the present invention, reference may behad to the accompanying drawing wherein like reference numeralsdesignate like parts and wherein:

FIGURE 1 is an operational diagram of one form of the method of thisinvention, employed for producing improved ceramic resistance elements;

FIGURE 2 is a grossly enlarged sectional view of a 4 ceramic resistanceelement made in accord with the present invention; and

"FIGURE 3 is a grossly enlarged sectional view of another embodiment ofa ceramic resistance element of the present invention.

Referring to FIGURE 1, the resistance composition of the presentinvention for producing ceramic resistance elements comprises theselected metal oxide or oxides, and finely divided glass particlessuspended in a vehicle, e.g., an organic screening agent, to form apaste which is applied onto a surface of a high-temperature-resistantelectrically nonconductive substrate or base 11. Preferably, the surfaceof the base should be lapped and polished to make it as smooth aspossible before the composition is applied, for as the surface becomessmoother the reproducibility of the electrical characteristics improves.Lapping and polishing the surface of the substrate is especiallyadvantageous in the case of resistance elements for variable resistorsand, for this purpose, it may also be desirable to polish the surface ofthe fired-on film.

Whenever a fixed volumetric resistance element is desired, such elementis produced by coating a substrate, e.-g., a cylindrical substrate 21,or by adding an electrically nonconductive refractory material to themixture for forming the resistance element into a desired shape asthoroughly disclosed in our above-menitoned co-pending applicationandconnecting not shown terminals to opposite ends of the element in asuitable manner.

The formula for the glass frit used in the practice of the presentinvention may be any one of several ordinarily used in this art, anexample thereof having a softening temperature of approximately 750 C.is as follows:

Percent PbO 63 B 0 25 SiO 12 The finely divided oxide of ruthenium oriridium oxide is mixed with the finely divided glass particles in thefollowing proportions (percentages by weight):

The metal oxide-glass mixture is combined with a vehicle, such as ethylcellulose dissolved in acetone-toluene if the mixture is being preparedfor application onto a flat or cylindrical surface. Varying amounts ofthe vehicle may be used, depending upon the consistency desired. A ratioof about one part of the metal oxide-glass mixture to three or fourparts of the vehicle forms a suitable resistance composition having theproper viscosity. The resistance composition is then deposited onto thesurface of a substrate and fired at suitable temperatures ranging from500 C. to 1000 C. to fuse the glass particles into a glass matrix, thetemperature depending upon the melting point of the glass employed inthe composition.

The desired resistance for the finished unit can be obtained by simplycontrolling the percent of the oxide of ruthenium and/ or iridium oxideand of the vehicle in the resistance com-position without bringing aboutan abrupt and unpredictable change in the sheet resistance, the percentof the vehicle generally determining the thickness of the fired ceramicresistance element unless the thickness of the unfired film is altered.I

The following table presents pertinent data of a number of differentceramic resistance elements made in accord with this invention, eachelement having a thickness in the range of about .0008 to .0011 which isapproximately 5 equal to .001 inch. The permissible thickness of thefilm can range from about .0002 to about .003 inch.

Percent Sheet Re- TC R. Per- Example Percent Metal Oxide Glass sistancecentl" C.

ohms/square G 65 0.01 82 81 0. 007 74 24.8 0.007 80 1. 300 0. 007 80 2,860 0. 002 2 s0 5. 700 0. 006 1. -RuOz/4.1-IrO 94.3 6,1 0 (.008 91 i9,000 O. 003 93. 2 36, 000 0. 0 90 49. 200 0. 004 90 72. 400 0. 007 95. 5128. 000 0. 005 95. 5 181, 000 0. (it

When 110 is substituted for RuO ceramic sheet resistance elements havinga higher resistance are produced, for instance, Examples 6 and 13 havethe same percent of metal oxide as Examples 5 and 12 respectively,however, the resistance per square is substantially higher. Other valuesof sheet resistance can be obtained by altering the percent of thevehicle in the composition, i.e., if the per cent of the vehicle isincreased without changing the thickness of the unfired film, thethickness of the fired film is decreased. Such changes account for theincrease in sheet resistance for Examples 5 and 11 when compared toExamples 4 and respectively. Other sheet resistance values may beobtained by combining various percentages of R110 and IrO as well asaltering the percent of the vehicle as in Examples 7 and 9.

In order to control the stability of the ceramic resistance elements,small percentages of cupric oxide and/ or manganese oxide are preferablydissolved in the glass as thoroughly disclosed in the Holmes c-opendingapplication entitled Precision Resistance Element, Serial No. 276,064,filed on April 26, 1963, and now abandoned, and assigned to the sameassignee as the present invention.

An example of the production of a complete ceramic resistance element isdescribed below. Twenty grams of RuO (particle size less than 325 mesh)and 80 grams of powdered glass frit (particle size less than 325 mesh,lead borosilicate frit 63% PhD, 25% B 0 and 12% SlOg) are mixed with aliquid, e.g., water, to form a slurry which is ground in a ball mill fortwo hours. The slurry after ball milling is of a homogeneousconsistency. The liquid is evaporated and the dry mixture of powderedglass frit and RuO is combined with a vehicle, for example, a solutionof ethyl cellulose in acetone-toluene. About 400 grams of the vehicleare mixed with 100 grams of the dry mixture of powdered glass frit andRuO to form a resistance composition which is mixed by suitable meansuntil a homogeneous suspension is obtained. The composition is appliedonto a surface of a ceramic substrate forming a layer about 0.003 inchin thickness on the ceramic substrate. The substrate with the layer ofpowdered RuO powdered glass frit, and screening agent is then fired inan oxidizing atmosphere causing the orga-nic materials to be driven off.As a result, the glass matrix containing the conductive particles isfused to the surface of the substrate.

The powdered glass frit forming a component of the ceramic resistanceelement can be made of any suitable glass or vitreous material having asoftening point below the temperature at which the base or substratedeforms. For instance, it can be a borosilicate frit, lead borosilicatefrit, cadmium, barium, calcium, or other frit having the proper fusiontemperature and expansion coetficient. The preparation of such frits iswell known and consists, e.g., of melting together boric oxide, silicondioxide and lead oxide, cadmium oxide, or barium oxide and pouring suchmolten composition into water to form the glass frit. The batchingredients may, of course, be any compound that will yield the desiredoxides under the fusing conditions of frit production, i.e., boric oxidewill be obtained from boric acid, silicon dioxide will be produced fromfiint, lead oxide will be produced from red lead or white lead, bariumoxide will be produced from barium carbonate, etc. The coarse glass fritis preferably milled for 2 to 20 hours, e.g., in a ball mill with water.

Instead of homogenizing a mixture of glass frit, an oxide of rutheniumand/or iridium oxide, and a vehicle to produce a suspension which issuitable for forming a fixed volumetric or thin film ceramic resistanceelement, organo metallic compounds of ho RuO or mixtures of the two maybe employed in conjunction with organo metallic compounds of fritforming materials to produce ceramic resistance elements.

The vehicle can consist of most any of the wellknown organic compoundswhich are capable of being completely volatilized or decomposed by heat.Preferably the vehicle should be able to keep the finely divided glassand metal oxide particles in suspension after the mixture has beendeposited onto the base. One such mixture of organic compounds whichwill maintain the particles in suspension is ethyl cellulose dissolvedin a trichloroethylenefenchone solution.

While there has been illustrated and described what is at presentconsidered to be a preferred embodiment of the present invention and amethod of making the same and a single modification thereof, it will beappreciated that numerous changes and modifications are likely to occurto those skilled in the art, and it is intended in the appended claimsto cover all those changes and the modifications which fall within thetrue spirit and scope of the present invention.

The invention claimed is:

1. A composition adapted to be applied onto a hightemperature-resistant,electrically nonconductive substrate and fired to form an electricalresistance element comprising 2-70 percent by weight of a finely dividedmetal oxide selected from the group consisting of RnO and IrO and 98-30percent by weight powdered glass frit.

2. A composition adapted to be applied to and fired on ahigh-temperature-resistant, electrically nonconductive substrate to forman electrical resistance element comprising 4-50 percent by weight of afinely divided metal oxide selected from the group consisting of RuO andIrO and 96-50 percent by weight powdered glass frit.

3. An electrical resistance element of the type wherein the resistancepath is a film of glass having a conductor in a fine state ofsubdivision dispersed therein as an ingredient, fired onto a base ofhigh-temperature-resistant, electrically non-conductive material,wherein the conductor is an oxide of a metal selected from the groupconsisting of Ru and Ir.

4. A resistance element com-prising a high-temperatureresistantelectrically nonconductive substrate having fired thereon a film ofresistance material comprising a solid-ified glass, and a finely dividedmetal oxide taken from the group consisting of R and IrO dispersed as aningredient throughout the solidified glass in electrically conductiverelationship.

5. A resistance element comprising a high-temperatureresistant,electrically nonconductive base having fired thereon a film ofresistance material comprising 98-30 percent by weight of solidifiedglass, and 2-70 percent by weight of at least one oxide of a metalselected from the group consisting of Ru and Ir in finely divided formdispersed throughout the solidified glass in electrically conductiverelationship.

6. A resistance element comprising an electrically nonconductive basehaving disposed on the surface thereof a composition comprising between2-70 percent by weight of at least one oxide of a metal selected fromthe group consisting of Ru and Ir in finely divided form dispersed in aglass matrix.

7. A resistance element comprising an electrically nonconductive basehaving disposed on the surface thereof, a film comprising between 2-70percent by weight of at least one oxide of a metal selected from thegroup consisting of Ru and Ir in finely divided form dispersed in aglass matrix, the thickness of the film being in the range of.0O0'2-.003 inch..

8. A resistance elementcomprising a high-temperatureresistant,electrically nonconductive base having fired thereon a film ofresistance material comprising 9830 percent by weight of solidifiedglass, and 2-70 percent by weight of an oxide of Ru in finely dividedform dispersed throughout the solidified glass.

9. A resistance element comprising a high-temperatureresistant,electrically nonconducting base having fired thereon a film ofresistance material comprising 98-30 percent by weight of solidifiedglass, and 2-70 percent by weight of an oxide of Ir in finely dividedform dispersed throughout the solidified glass.

10. A resistance element comprising a high-temperature-resistant,electrically nonconductive base having fired thereon a film ofresistance material comprising 98-30 percent by Weight of solidifiedglass, and 27() percent by weight of an oxide of Ru in finely dividedform dispersed throughout the solidified glass, the film having athickness in the range of 0002-003 inch.

11. A resistance element comprising a high-temperature-resistant,electrically nonconductive base having fired thereon a film ofresistance material comprising 98-30 percent by weight of solidifiedglass, and 270 percent by Weight of an oxide of Ir in finely dividedform dispersed throughout the solidified glass, the film having athickness in the range of 0002-.003 inch.

References Cited by the Examiner UNITED STATES PATENTS 3,052,573 9/1962Dumesnil 117-221 3,149,002 9/1964 Place et al ll7-227 ALFRED L. LEAVITT,Primary Examiner.

RICHARD D. NEVIUS, Examiner.

W. L. JARVIS, Assistant Examiner.

1. A COMPOSITION ADAPTED TO BE APPLIED ONTO A HIGHTEMPERATURE-RESISTANT,ELECTRICALLY NONCONDUCTIVE SUBSTRATE AND FIRED TO FORM AN ELECTRICALRESISTANCE ELEMENT COMPRISING 2-70 PERCENT BY WEIGHT OF A FINELY DIVIDEDMETAL OXIDE SELECTED FROM THE GROUP CONSISTING OF RU/2 AND IRO2, AND98-30 PERCENT BY WEIGHT POWDERED GLASS FRIT.